Description of the Prior Art
French patent application number 78.36248 filed Dec. 22nd, 1978 in the name of Thomson-CSF describes coaxial resonant cavities for grid vacuum tubes, in which the cavities are constituted by a coaxial line separated into a plurality of elementary resonators by pistons. The cavity-separating pistons may also provide coupling for the cavities. This coupling may be capacitive (see FIG. 2 of the above-mentioned patent application), or it may be inductive (see FIG. 4 thereof). The coupling may be adjustable; for example FIG. 3 of the above-mentioned patent application shows how an adjustable capacitive coupling may be obtained by using a piston having two parts A and B which are nested one in the other. Further, by changing the position of the pistons along the line by means of a system of slots and lugs, or by means of rods, the cavities can be frequency tuned.
These coaxial cavities are coupled to the input circuit or to the output circuit of the grid vacuum tube, eg. in tubes which are used in power amplifiers for TV transmitters which are required to meet precise passband specifications.
For safety reasons, it is general practice to ground both of the cylinders which constitute the coaxial line.
The cylinders are then capacitively decoupled from two electrodes of the tube in order to DC isolate the cylinders from the electrodes. Such capacitive decoupling is generally performed by means of an insulating sheet which is clamped between two cylindrical members.
In the prior art, capacitive decoupling between the cylinders and the electrodes is provided at the tube ends of the cylinders, thus grounding the cylinders over their entire length.
A problem arises at the highest frequencies at which the cavities operate, in which the piston of the first cavity starting from the tube end is situated close to the tube ends of the cylinders. The pistons are located at voltage nodes in the system of standing waves which is established in the line, and hence at current antinodes.
The insulating sheet which provides capacitive decoupling between the inside cylinder and one of the tube electrodes is thus situated in a high current zone. Losses in the dielectric are high. Such losses, in addition to constituting a drawback in their own right, additionally give rise to increased temperatures in a location which is very difficult to cool.
Preferred embodiments of the present invention solve this problem.